Many power electronic converters such as frequency converters, for example, include a converter bridge adapted to transfer electrical energy between the AC mains and the DC terminal of the converter. Said DC terminal can be represented by the positive and negative rails in the intermediate DC link of a frequency converter, for instance. To make the charging of the intermediate DC link easier, the converter bridge often includes controlled semiconductor components such as thyristors or transistors, for example. The converter bridge may also be a half-controlled bridge in which only one branch, connected with the positive or negative pole of said DC terminal, includes controlled semiconductor components, while the opposite branch includes diodes. Diodes and other semiconductor components have to be protected against potential overvoltage transients occurring in the AC mains. Thyristors, for instance, have a turn-on delay of 2 μs, approximately, during which time the forward voltage in the thyristor may rise detrimentally high even if a firing pulse were continuously delivered to the thyristor. If the circuit connected with the thyristor has a small inductance and the forward voltage of the thyristor is too high at the turn-on moment, the rate of change of current (di/dt, A/s) may become so high that the thyristor becomes damaged. For diodes, on the other hand, a typical damaging mechanism is a breakdown caused by a reverse voltage too high.
In a prior-art solution, AC chokes are placed between the AC mains and the converter bridge to filter out overvoltage transients occurring in the AC mains and to restrict the rates of change of current in the semiconductor components in the converter bridge. A drawback of said AC chokes is that they increase the load-dependent voltage drop of the DC voltage in the DC terminal. In the case of a diode and/or thyristor bridge, for example, said AC chokes increase the commutation angle and thus decrease the DC voltage and affect the power factor at the DC terminal.